Switches are a useful building block that finds applications in many types of integrated circuits. When combined with a capacitor to form a switched capacitor they can be used to provide a circuit having resistive electrical characteristics. Common uses for switched capacitors include switched capacitor filters, integrators, sigma-delta modulators for analog-to-digital and digital-to-analog converters, and others.
Switched capacitors are efficiently implemented using MOSFET transistors. In fact, switched capacitor circuits have become much more popular with the increasing use of MOSFETs in analog and mixed signal applications.
Switches formed using one device typically have a first source-drain region connected to the input, a second source-drain region connected to the output, and a gate that is selectively tied to one of two voltage levels, for example, it may be grounded or connected to a supply voltage. In this configuration, the gate-to-source voltage of the transistor forming the switch is dependent on the input voltage. When the switch is an n-channel transistor and is conducting, the gate-to-source voltage decreases with increasing input voltage.
This change in gate-to-source voltage changes the series resistance of the switch, that is, the resistance of the switch is dependent on the input voltage begin switched. This nonlinearity can limit circuit performance in many applications. Accordingly, bootstrapped switches are used, where the gate voltage for the switch increases with input voltage such that the switch's gate-to-source voltage is maintained.
This configuration requires that a transistor used to discharge the switch's gate voltage to ground sustain a very high drain-to-source voltage. This can create hot-electron problems, which can reduce the discharging transistor's lifetime. For example, hot carriers can be injected into a gate of a transistor thereby changing the transistor's threshold voltage over time.
Accordingly, what is needed are circuits, methods, and apparatus that provide bootstrapped switches having improved reliability.